Femtosecond electronic relaxation and real-time vibrational dynamics in 2′-hydroxychalcone

Abstract

Femtosecond ultrafast electronic relaxation and vibrational dynamics in 2′-hydroxychalcone after deep ultraviolet (DUV) excitation were observed by two types of pump–probe spectroscopy experiments, i.e., DUV-pump pulse and visible-broadband-probe pulse (DUV/Vis) experiments and DUV-pump and DUV-probe (DUV/DUV) pulse experiments. Time-dependent density functional theory (TDDFT) calculations were performed to elucidate relaxation dynamics from the third singlet electronic excited state S₃. The DUV/Vis experiments and TDDFT calculations have disclosed the ultrafast dynamics of internal conversion from the initial S₃ state (τ₁ ≈ 35 fs) to the S₁ state via a rapid process through the S₃/S₂ conical intersection and proton transfer [OH: τ₂(H) ≈ 93 and OD: τ₂(D) ≈ 164 fs] before deactivation through the S₁/S₀ conical intersection (τ₃ ≈ 690 fs). Thanks to the ultrashort pump and probe pulses, real-time observation of vibrational modes coupled to the electronic excitation was realized providing both amplitudes and phases. Spectrogram analyses were performed based on the real-time spectra obtained by the DUV/Vis experiments, in which instantaneous vibrational frequencies reflecting molecular structural change after the impulsive excitation were visualized. The vibrational frequency of central CC bond stretch decreases from ∼1600 cm⁻¹ to ∼1560 cm⁻¹ in about 200–500 fs and recovers in ∼550 fs. Normal mode analyses along the decay path support the observed variation of the CC stretching frequency. The temporal weakening of the central CC bond is connected with the angle of the two aromatic rings which flip back to the initial conformation.